Refining apparatus and processes

ABSTRACT

This invention relates to melting and refining apparatus and processes, and is concerned with the problem of obtaining the correct metallurgical conditions in coreless induction furnaces. The invention consists in using a furnace with a single work coil connected through switching means to one or more electric power sources arranged so that the frequency applied to the single work coil can be varied. The electric power may be supplied by a single variable frequency power source such as a static thyristor inverter device, or by a number of individual sources, each designed to produce electric currents alternating at a particular frequency. This arrangement enables material to be melted by supplying current to the work coil at high power and a high frequency and to be refined by using a low power and a low frequency.

United States Patent [191 Laflin [451 Feb. 18,1975

REFINING APPARATUS AND PROCESSES Filed: May 24, 1973 Appl. No.: 363,351

[30] Foreign Application Priority Data May 26, i972 Great Britain 24928/72 US. CLl 13/27, 219/1077 Int. Cl. HOSb 5/16 Field of Search..... 13/26, 27; 219/1075, 10.77

Primary Examiner-R. N. Envall, Jr. Attorney, Agent, or Firmlrving M. Weiner [57] ABSTRACT This invention relates to melting and refining apparatus and processes, and is concerned with the problem of obtaining the correct metallurgical conditions in coreless induction furnaces.

The invention consists in using a furnace with a single work coil connected through switching means to one or more electric power sources arranged so that the frequency applied to the single work coil can be varied. The electric power may be supplied by a single variable frequency power source such as a static thyristor inverter device, or by a number of individual sources, each designed to produce electric currents alternating at a particular frequency. This arrangement enables material to be melted by supplying current to the work coil at high power and a high frequency and to be refined by using a low power and a low frequency.

13 Claims, 2 Drawing Figures 2 6 f5 2 21 II I III 50 #2 500 #2 POM/19 6001905 POM/1? SOURCE F/lPAC/IUR 0M7 owe/rap u/v/r REFINING APPARATUS AND PROCESSES This invention relates to melting and refining apparatus and processes, and is concerned with the problem of obtaining the correct metallurgical conditions in coreless induction furnaces.

BACKGROUND OF THE INVENTION It is known that, in order to produce ideal refining conditions in a bath of molten metal, it is desirable that t a vigorous agitation of the metal should be obtained. This vigorous agitation can be obtained by the use of a relatively low-frequency alternating current. The optimum frequency in any particular case depends on a number of factors and, in particular, on the furnace capacity. Thus, for example, optimum conditions may be produced in a furnace of 500 pounds capacity by using a frequency of approximately 500 Hz, whereas a furnace having 2,000 pounds capacity might require a frequency of I Hz and a furnace of 6,000 pounds capacity a frequency of 50 Hz.

It is also known that the use of relatively low frequencies imposes a limit on the power that can be applied to a furnace work coil of a given size and increases the time required for the initial melting operation. The use of low frequencies also make it necessary to use charge material of large cross-sectional area and, in some cases, efficient metling can only be obtained by using liquid metal in the charage.

SUMMARY OF THE INVENTION The present invention provides a coreless induction furnace comprising a refractory lining, and a single work coil surrounding the refractory lining. The single work coil is provided with first and second terminals, one at each end of the coil. There is also provided a variable frequency electric power source, and a plurality of power factor capacitor units. There is also provided switching means for connecting the first and second terminals of the work coil to a selected one of the power factor capacitor units and to the output of the variable frequency electric power source.

The present invention also provides a coreless induction furnace comprising a refractory lining, and a single work coil surrounding the refractory lining. The single work coil is provided with first and second terminals, one at each end of the work coil. There is also provided a plurality of electric power sources producing electric currents alternating at mutually different frequencies. The apparatus also includes a plurality of power capacitor units, one associated with each of the power sources. There is also included switching means for connecting the first and second terminals of the work coil to a selective one of the power sources and the associated power factor capacitor unit.

The present invention also provides a refining process carried out in a coreless induction furnace having a single work coil, comprising the steps of placing a charge to be melted and refined in the coreless induction furnace, supplying electric current to the single work coil at a relatively high power and a relatively high frequency to melt the charge, and supplying electric current to the single work coil at a relatively low power and a relatively low frequency to refine the melt of said charge.

From one aspect the invention consists in a coreless induction furnace having a single work coil connected through switching means to a variable-frequency electric power source.

From another aspect the invention consists in a coreless induction furnace having a single work coil connected through changeover switching means to a plurality of electric power sources capable of producing electric currents alternating at mutually different frequencies.

From yet another aspect the invention consists in a refining process carried out in a coreless induction furnace having a single work coil, wherein a charge is melted by supplying electric current to said work coil at relatively high power and a relatively high frequency, and wherein the melt is refined by supplying electric current to said coil at a relatively low power and a relatively low frequency.

Use of apparatus in accordance with the last paragraph but two enables the frequency of the current applied to the work coil to be adjusted to the optimum value for all stages of a refining process. Thus, in a process in accordance with the invention using a variablefrequency electric power source, it is possible to start the initial melting cycle at a relatively high power and a relatively high frequency. The power density at this stage may be, for example, of the order of 800 to 1,000 Kw per ton while the frequency may be, for example, in the range 700 to 10,000 Hz. As the charge becomes molten, the frequency and the power can be gradually reduced until optimum refining conditions are obtained. During this stage of the process alloying elements, and possibly swarf or borings, may be added to the molten bath. The refining process may be carried out in air or in a vacuum in accordance with the particular conditions required.

Finally, it may be desirable to increase the frequency towards the end of the refining process in order to reduce turbulence and atmospheric contamination of the metal.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a block circuit diagram of a first embodiment of the invention showing a coreless induction furnace powered by a variable frequency power source.

FIG. 2 is a block circuit diagram of a second embodiment of the invention showing a coreless induction furnace powered by two alternative power sources.

DETAILED DESCRIPTION OF INVENTION FIG. 1 of the drawings shows a coreless induction furnace 1 having a refractory lining 2 and a single work coil 3 provided with terminals 4 and 5, one at each end of the coil. These terminals are connected through flexible, water cooled cables 6 to a furnace busbar system Three capacitor units 8 9 and 10 are connected to the busbar system 7 through respective double pole switches ll, 12 and 13. The capacitor unit 8 is suitable for use over a range of frequencies between 50 and 200 Hz., the capacitor unit 9 is suitable for use over a range of frequencies between 200 and 700 I-Iz., and the capacitor unit 10 is suitable for a range of frequencies between 700 and 10,000 Hz. The three capacitor units are connected through respective further double pole switches 14, 15 and 16 to a further busbar system 17. This busbar system 17 is connected through a main contactor 18 to the output of a variable frequency power source 19 drivin from a three-phase voltage supply connected to input terminals 20. The power source 19 may be, for example, a so-called static inverter which is constituted by means for rectifying the threephase input and deriving avariable frequency output from the resulting D.C. supply.

The switches 11 to 16 are preferably coupled to the control member for changing the frequency of the power source 19 so that the appropriate capacitor unit is automatically switched into circuit as the frequency of the power source is changed.

FIG. 2 of the drawings shows a coreless induction furnace 1 similar to that shown in FIG. 1 and connected in the same way by water-cooled cables 6 to a busbar system 7. This system 7 is connected through a pair of interlocked double pole switches 21 and 22 to capacitor units 23 and 24. It will be seen that the interlocking between the two switches 21 and 22 is such that the busbar 7 can only be connected to one of the capacitor units 23 and 24 at any one time. The capacitor unit 23 is suitable for use at a frequency of 50 Hz., and is connected through a pair of lines 27 to the output of'a 50 Hz. power source 25. This power source may include a static balancer for converting a high voltage, low power, three-phase supply 26 to the required single phase supply on the lines 27. It may also include a main contactor in the two-phase supply and a high tension breaker in the three-phase supply.

The capacitor unit 24 is suitable for use at a frequency of 500 Hz., and is connected through a pair of lines 28 to 500 Hz., power source 29. This power source 29 may be constituted by a static thyristor frequency converter connected to a medium voltage, high power three-phase supply 30. Alternatively, the power source 29 may include a rotary-type frequency converter comprising a motor driven by the three-phase suppply 30 and mechanically coupled to a single-phase, 500 Hz. generator feeding the lines 28. In this case, it will also include conventional contactors and motor starter.

The capacitor units 23 and 24 may be connected in series in the lines 27 and 28, respectively, or they may be connected in parallel with these lines.

Referring to FIG. 1, as the frequency of the power source is changed, it may be necessary to change the power factor capacitors connected either in series or in parallel between the power supply and the work coil. Thus, for example, three capacitor units may be provided in a typical installation and these units may be switched in automatically as the frequency is changed. One unit may be designed to cover a low frequency range, for example, from 50 to 200 Hz, one unit for an intermediate frequency range from 200 to 700 Hz, and one for a medium frequency range from 700 to 1,000 Hz. Preferably, mechanical and electrical interlocks are provided to ensure that the correct capacitor unit is connected in circuit at every frequency.

A suitable variable-frequency electric power source may be constituted by a static inverter device operating over a range of frequencies, for example, from 50 to 10,000 Hz. However, if a variable-frequency power source is not available, some of the advantages of the invention may still be realised by using a plurality of power sources of mutually different operating frequencies (see FIG. 2). Thus, in some cases, it may be sufficient to use two power sources, one designed to produce a relatively low-frequency alternating current and the other designed to produce a relatively highfrequency alternating current. The low-frequency source may, for example, operate at 50 Hz while the relatively high-frequency source operates at 500 Hz. The low-frequency source may, in this case, be powered from the supply mains without frequency conversion while the relatively high-frequency source includes a rotary-type frequency converter or a static thyristor frequency inverter. Where two or more separate power sources are used, each will be provided with its own capacitor rack, the value of which will be appropriate to the operating frequency of the source. The two or more power sources will be connected to the single work coil of the furnace through mechanically and electrically interlocked changeover switching means designed to ensure that only one of the.power sources is connected to the work coil at a time.

What we claim is:

1. A coreless induction furnace comprising, in combination:

a refractory lining;

a single work coil surrounding said refractory lining;

said single work coil having first and second terminals one at each end of said single work coil;

a variable frequency electric power source;

a plurality of power factor capacitor units; and

switching means for connecting said first and second terminals to a selected one of said power factor capacitor units and to the output of said variable frequency electric power source.

2. A furnace as claimed in claim 1, wherein said variable-frequency electric power source is constituted by a static inverter device operating over a range of frequencies between 50 and 10,000 Hz.

3. A coreless induction furnace comprising in combination:

a refractory lining;

a single work coil surrounding said refractory lining; said single work coil being provided with first and second terminals, one at each end of said work coil;

a plurality of electric power sources producing electric currents alternating at mutually different frequencies;

a plurality of power factor capacitor units, one associated with each of said power sources; and

switching means for connecting said first and second terminals to a selected one of said power sources and the associated power factor capacitor unit.

4. A furnaces as claimed in claim 3, wherein one of said electric power sources produces electric currents alternating at substantially 50 Hz while the other power source, or each of the other power sources, produces electric currents alternating at frequencies higher than 50 Hz.

5. A furnace as claimed in claim 4, wherein said one of said electric powers sources is powered from the supply main without frequency conversion.

6. A furnace as claimed in claim 4, wherein said other power source, or each of said other power sources, includes a rotary-type frequency converter. 4,

7. A furnace as claimed in claim 4, wherein said other power source, or each of said other power sources, includes a static thyristor frequency inverter.

A furn as c e r rnsla m h r in th Qapacitor units are provided, one of which is designed to cover a range of frequencies from 50 to 200 Hz, a sec 0nd one of which is designed to cover a range of frequencies from 200 to 700 Hz, and a third one of which is designed to cover a range of frequencies from 700 to 10,000 Hz.

9. A furnace as claimed in claim 8, including mechanical and electrical interlocks adapted to ensure that the correct capacitor unit is connected in circuit at each operating frequency.

10. A refining process carried out in a coreless induction furnace having a single work coil, comprising the steps of:

placing a charge to be melted and refined in said coreless induction furnace;

supplying electric current to said single coil at a relatively high power and a relatively high frequency to melt said charge; and

supplying electric current to said single work coil at a relatively low power and a relatively low power and a relatively low frequency to refine the melt of said charge.

11. A process as claimed in claim 10, including the 5 and atmospheric contamination of said melt. 

1. A coreless induction furnace comprising, in combination: a refractory lining; a single work coil surrounding said refractory lining; said single work coil having first and second terminals one at each end of said single work coil; a variable frequency electric power source; a plurality of power factor capacitor units; and switching means for connecting said first and second terminals to a selected one of said power factor capacitor units and to the output of said variable frequency electric power source.
 2. A furnace as claimed in claim 1, wherein said variable-frequency electric power source is constituted by a static inverter device operating over a range of frequencies between 50 and 10,000 Hz.
 3. A coreless induction furnace comprising in combination: a refractory lining; a single work coil surrounding said refractory lining; said single work coil being provided with first and second terminals, one at each end of said work coil; a plurality of electric power sources producing electric currents alternating at mutually different frequencies; a plurality of power factor capacitor units, one associated with each of said power sources; and switching means for connecting said first and second terminals to a selected one of said power sources and the associated power factor capacitor unit.
 4. A furnaces as claimed in claim 3, wherein one of said electric power sources produces electric currents alternating at substantially 50 Hz while the other power source, or each of the other power sources, produces electric currents alternating at frequencies higher than 50 Hz.
 5. A furnace as claimed in claim 4, wherein said one of said electric powers sources is powered from the supply main without frequency conversion.
 6. A furnace as claimed in claim 4, wherein said other power source, or each of said other power sources, includes a rotary-type frequency converter. 4,
 7. A furnace as claimed in claim 4, wherein said other power source, or each of said other power sources, includes a static thyristor frequency inverter.
 8. A furnace as calimed in claim 1, wherein three capacitor units are provided, one of which is designed to cover a range of frequencies from 50 to 200 Hz, a second one of which is designed to cover a range of frequencies from 200 to 700 Hz, and a third one of which is designed to cover a range of frequencies from 700 to 10,000 Hz.
 9. A furnace as claimed in claim 8, including mechanical and electrical interlocks adapted to ensure that the correct capacitor unit is connected in circuit at each operating frequency.
 10. A refining process carried out in a coreless induction furnace having a single work coil, comprising the steps of: placing a charge to be melted and refined in said coreless induction furnace; supplying electric current to said single coil at a relatively high power and a relatively high frequency to melt said charge; and supplying electric current to said single work coil at a relatively low power and a relatively low power and a relatively low frequency to refine the melt of said charge.
 11. A process as claimed in claim 10, including the step of supplying a current to said single work coil at a frequency in the range 700 to 10,000 Hz, and at a power density between 800 and 1000 Kw per ton to melt said charge.
 12. A process as claimed in claim 10, including the step of increasing the operating frequency towards the end of the refining step in order to reduce turbulence and atmospheric contamination of said melt.
 13. A process as claimed in claim 11, including the step of increasing the operatinG frequency towards the end of the refining step in order to reduce turbulence and atmospheric contamination of said melt. 